This invention relates to an apparatus to perform the four point flexure mechanical test for high performance ceramics at ambient conditions. The four point flexure test, also known as the modulus of rupture or bend strength test, is used to measure the tensile strength of a ceramic. The flexure test can be used for quality control or design data generation purposes. For either application (but especially for design data), it is important to perform the test in as error-free a fashion as possible. This aspect is of increasing importance as new advanced or high performance ceramics are employed in structural applications. Considerably greater care must be exercised in testing these materials as opposed to earlier methods developed for whitewares or refractory insulation.
In the four point flexure test, a prismatic specimen (usually rectangular or square) is placed in a jig or fixture which has four knife edges or load application bearings to apply load to the specimen. These loading points are arranged in a symmetrical fashion and create bending stresses and strains in the ceramic specimen. It is essential that these loading points apply even, uniform loadings to the specimen. This is generally not a problem with well-machined ceramic specimens, but can be a problem with as-fired or sintered ceramic specimens, that are slightly irregular geometrically. The specimen is loaded to failure and simple beam theory is used to calculate the maximum tensile strength in the specimen at the instant of failure. The strength numbers will be accurate and precise only if the loading points apply even, uniform loads to the specimen. A standard procedure for flexure testing has been developed: US Army Military Standard MIL-STD 1942 (MR) dated 21 November 1983 and entitled "Flexural Strength At High Performance Ceramics At Ambient Temperature." This method was developed to create a standard practice that will create error-free test results suitable for high strength, brittle ceramics.